The effects of Mach number on turbulence behaviors are investigated by means of direct numerical simulation of compressible turbulent boundary layers with the free-stream Mach numbers 2,4 and 6.Some typical turbulent quantities,including the velocity and vorticity fluctuations,intermittency,sweep and ejection events and turbulent kinetic energy budget,are analyzed.The turbulence intensities are significantly decreased with the increasing Mach number.The onset of intrinsic intermittency occurs nearer the wall for a higher Mach number.The ejection event is more frequent and the sweep event becomes less frequent.Moreover,the magnitudes of turbulent kinetic energy production,diffusion and transport terms are increased and the solenoidal dissipation is slightly decreased as the Mach number is increased.
The interaction of strain and vorticity in compressible turbulent boundary layers at Mach number 2.0 and 4.9 is studied by direct numerical simulation(DNS)of the compressible Navier-Stokes equations.Some fundamental characteristics have been studied for both the enstrophy producing and destroying regions.It is found that large enstrophy production is associated with high dissipation and high enstrophy,while large enstrophy destruction with moderate ones.The enstrophy production and destruction are also correlated with the dissipation production and destruction.Moreover,the enstrophy producing region has a distinct tendency to be‘sheet-like’structures and the enstrophy destroying region tends to be‘tube-like’in the inner layer.Correspondingly,the tendency to be‘sheet-like’or‘tube-like’structures is no longer obvious in the outer layer.Further,the alignment between the vorticity vector and the strain-rate eigenvector is analyzed in the flow topologies.It is noticed that the enstrophy production rate depends mainly on the alignment between the vorticity vector and the intermediate eigenvector in the inner layer,and the enstrophy production(destruction)mainly on the alignment between the vorticity vector and the extensive(compressive)eigenvector in the outer layer.
Nitrogen injection under conditions close vicinity of the liquid-gas critical point is studied numerically.The fluid thermodynamic and transport properties vary drastically and exhibit anomalies in the near-critical regime.These anomalies can cause distinctive effects on heat-transfer and fluid-flow characteristics.To focus on the influence of thermodynamics on the flow field,a relatively low injection Reynolds number of 1 750 is adopted.For comparisons,a reference case with the same configuration and Reynolds number is simulated in the ideal gas regime.The model accommodates full conservation laws,real-fluid thermodynamic and transport phenomena.Results reveal that the flow features of the near-critical fluid jet are significantly different from their counterpart.The near-critical fluid jet spreads faster and mixes more efficiently with the ambient fluid along with a more rapidly development of the vortex pairing process.Detailed analysis at different streamwise locations including both the flat shear-layer region and fully developed vortex region reveals the important effect of volume dilatation and baroclinic torque in the near-critical fluid case.The former disturbs the shear layer and makes it more unstable.The volume dilatation and baroclinic effects strengthen the vorticity and stimulate the vortex rolling up and pairing process.
A vortex ring impacting a three-dimensional bump is studied using large eddy simulation for a Reynolds number Re=4×104 based on the initial diameter and translational speed of the vortex ring.The effects of bump height and vortex core thickness for thin and thick vortex rings on the vortical flow phenomena and the underlying physical mechanisms are investigated.Based on the analysis of the evolution of vortical structures,two typical kinds of vortical structures,i.e.,the wrapping vortices and the hair-pin vortices,are identified and play an important role in the flow state evolution.The boundary vorticity flux is analyzed to reveal the mechanism of the vorticity generation on the bump surface.The circulation of the primary vortex ring reasonably elucidates some typical phases of flow evolution.Further,the analysis of turbulent kinetic energy reveals the transition from laminar to turbulent state.The results obtained in this study provide physical insight into the understanding of the mechanisms relevant to the flow evolution and the flow transition to turbulent state.
Characteristic boundary conditions that are capable of handling general fluid mixtures flow at all flow speeds are developed.The formulation is based on fundamental thermodynamics theories incorporated into an efficient preconditioning scheme in a unified manner.Local one-dimensional inviscid(LODI)relations compatible to the preconditioning system are proposed to obtain information carried by incoming characteristic waves at boundaries accurately.The approach has been validated against a variety of sample problems at a broad range of fluid states and flow speeds.Both acoustic waves and hydrodynamic flow features can pass through the boundaries of computational domain transparently without any unphysical reflection or spurious distortion.The approach can be reliably applied to fluid flows at extensive thermodynamic states and flow speeds in numerical simulations.Moreover,the use of the boundary condition shows to improve the computational efficiency.
Numerical investigation of a transverse sonic jet injected into a supersonic crossflow was carried out using large-eddy simulation for a free-stream Mach number M = 1.6 and a Reynolds number Re = 1.38×10~5 based on the jet diameter.Effects of the jet-to-crossflow momentum ratio on various fundamental mechanisms dictating the intricate flow phenomena,including flow structures, turbulent characters and frequency behaviors,have been studied.The complex flow structures and the relevant flow features are disc...
Guo-Lei Wang~(a)) and Xi-Yun Lu~(b)) Department of Modern Mechanics,University of Science and Technology of China,Hefei,Anhui 230026,China
Large-eddy simulation of a sonic injection from circular and elliptic injectors into a supersonic crossflow has been performed.The effects of injector geometry on various fundamental mechanisms dictating the intricate flow phenomena including shock/jet interaction,jet shear layer vortices and their evolution,jet penetration properties and the relevant turbulence behaviors have been studied systematically.As a jet issuing transversely into a supersonic crossflow,salient three-dimensional shock and vortical structures,such as bow,separation and barrel shocks,Mach disk,horseshoe vortex,jet shear layer vortices and vortex pairs,are induced.The shock structures exhibit considerable deformations in the circular injection,while their fluctuation becomes smaller in the elliptic injection.The jet shear layer vortices are generated at the jet periphery and their evolution characteristics are analyzed through tracing the centroid of these coherent structures.It is found that the jet from the elliptic injector spreads rapidly in the spanwise direction but suffers a reduction in the transverse penetration compared to the circular injection case.The turbulent fluctuations are amplified because of the jet/crossflow interaction.The vertical Reynolds normal stress is enhanced in the downstream of the jet because of the upwash velocity induced by the counter-rotating vortex pair.
Turbulent channel flows with consideration of the buoyancy effect of the bubble phase is investigated by means of the Direct Numerical Simulation(DNS).This two-phase system is solved by a two-way coupling Lagrangian-Eulerian approach.The Reynolds number based on the friction velocity and the half-width of the channel is 194,and the gravitational acceleration varies from-0.5 to 0.5,ranging from the upflow to the downflow cases.This study aims to reveal the influence of buoyancy on the turbulence behavior and the bubble motion.Some typical statistical quantities,including the averaged velocities and velocity fluctuations for the fluid and bubble phases,as well as the flow structures of the turbulence fluctuations,are analyzed.
A vortex ring impacting a three-dimensional circular cylinder is studied using large eddy simulation(LES) for a Reynolds number Re = 4 × 10 4 based on the initial translation speed and diameter of the vortex ring.We have investigated the evolution of vortical structures and identified three typical evolution phases.When the primary vortex closely approaches to the cylinder,a secondary vortex is generated and its segment parts move inward to the primary vortex ring.Then two large-scale loop-like vortices are formed to evolve in opposite directions.Thirdly,the two loop-like vortices collide with each other to form complicated small-scale vortical structures.Moreover,a series of hair-pin vortices are generated due to the stretching and deformation of the tertiary vortex.The trajectories of vortical structures and the relevant evolution speeds are analyzed.The total kinetic energy and enstrophy are investigated to reveal their properties relevant to the three evolution phases.
